1. Technical Field
The present invention is directed to a semiconductor device and a process for fabricating the device.
2. Art Background
A number of semiconductor devices have a structure in which two electrodes are separated by a layer of dielectric material. For example, a MOSFET (metal-oxide-semiconductor field effect transistor) device is a voltage-controlled device that has a channel region linking two electrically-separated, highly conductive regions of a first conductivity type diffused into an opposite conductivity type semiconductor substrate. When a voltage that is above a certain threshold voltage (VTH) is applied to a gate electrode overlying but insulated from the channel region by a gate dielectric material, current flows from one highly conductive region (the source) to the second highly conductive region (the drain).
The voltage that is applied to the device causes one electrode (the cathode) to inject electrons across the dielectric layer and into the other electrode (the anode). One skilled in the art will understand that, in these devices, whether an electrode functions as an anode or a cathode depends upon the conditions (i.e. the direction of electron flow) under which the device is operated. It is also understood that the electrodes are electrically conductive in that they contain free carriers.
The electrons are injected into the anode with high energies (i.e. energies that are above the conduction band edge of the anode material). Such high energies are typically above about 0.2 eV. The electron injection energy (Ej) is defined herein with respect to the conduction band edge of the anode material. High-energy electrons in the anode undergo impact ionization, which creates holes. Electron impact ionization is the process in which a high-energy electron collides with a valence electron. This collision promotes the valence electron into a higher energy empty state either in the valence band or the conduction band. The promotion of the valence electron leaves behind an energetic hole in the valence band. An energetic hole has an energy that is typically 0.2 eV greater than the valence band edge in the anode material. As defined herein, hole injection or anode injection refer to energetic hole injection from the anode into the dielectric material. Thus the product of impact ionization is at least one energetic hole. The rate of impact ionization is a strong function of Ej (decreasing as Ej decreases) and of the band gap of the anode material (decreasing as the band gap increases).
When such energetic holes are generated in the anode, there is a tendency for the holes to be injected from the anode, into the dielectric layer. Hole injection into the gate dielectric from the anode has been observed to damage the dielectric layer. When holes are injected into the dielectric layer, they cause the properties of the dielectric layer to change. Such changes in the dielectric material include an increase in the number of interface states, an increase in the trapped charge, an increased number of leakage sites, and mechanical and chemical changes. Such changes potentially degrade the device performance in a variety of ways. Such degradation is manifested by reduced subthreshold slope, transconductance degradation, stress induced leakage current, threshold voltage drift (VTH drift) and dielectric breakdown. Since devices are designed to operate with specific characteristics, these changes can compromise device performance and eventually may cause device failure. Accordingly, devices in which the gate dielectric is less susceptible to damage are sought.
The present invention is directed to a semiconductor device in which a dielectric material is interposed between two electrodes. Examples of such devices include active devices such as metal oxide semiconductor field effect transistors (MOSFET), charge-coupled devices, and floating gate memory devices (e.g. EEPROM devices) and passive devices such as capacitors. The device of the present invention is designed to reduce the amount of damage to the dielectric material of the device during device operation. By reducing the amount of damage to the dielectric material over time, the lifetime of the device is extended.
As previously noted, dielectric damage has been correlated to the injection of holes into the dielectric material from the anode. The present invention is a semiconductor device wherein the number of holes that penetrate the dielectric (typically SiO2) material is reduced (compared to prior art devices) over time. The rate at which the dielectric is damaged by hole penetration is commensurately reduced. The number of holes that penetrate the dielectric are controlled by selecting an electrode material that does one or more of the following: 1.) reduces the number of electrons incident from the cathode to the anode; 2.) reduces the probability that electrons incident on the anode will produce holes by undergoing impact ionization; and 3.) reduces the total energy of the holes produced by impact ionization (thereby decreasing the probability that the holes will be injected from the anode into the dielectric material).
This result is accomplished by a layer of material with a bandgap (i.e., the energy difference between the valence band edge and the conduction band edge of the material) higher than that of silicon (i.e. higher than 1.1 eV) that is adjacent to the dielectric layer. The high bandgap material is electrically conductive (i.e., it serves as a conducting electrode containing free carriers). In order for the high bandgap material to have the requisite conductivity, the high bandgap material is doped. The dopant can be either n-type or p-type, depending upon the nature of the materials adjacent to the high bandgap material and other device considerations. The doping results in a high bandgap material having a single conductivity type (either dominated by electrons or holes). In such a material, the majority dopant is either n-type (dominated by electrons) or p-type (dominated by holes). Consequently, the layer of high bandgap material functions as an electrode. Whether the high bandgap material functions as an anode or a cathode depends upon the operating conditions of the device. In the present invention, the electrode with the high band gap material is either the high band gap material alone or a composite electrode that has more than one material layer. In the composite electrode embodiments, the high bandgap material must be the material that is adjacent to (i.e. next to and in contact with) the dielectric layer.
The present invention is directed to any semiconductor device in which the dielectric material is between two electrodes. However, for convenience and clarity, the invention is described in terms of a MOSFET device. In the MOSFET device used for descriptive purposes herein, the dielectric layer is the gate dielectric layer. Thus, applicants refer to a MOSFET devices and gate dielectric layers for example only, and the invention is not to be construed as limited to MOSFET devices or gate dielectric layers.
In a typical MOSFET device, the gate dielectric layer is SiO2. SiO2 is susceptible to damage when penetrated by holes. SiO2 is one of a class of electrically insulating materials that have a relatively large bandgap. Hole penetration into such materials induces the formation of charge traps which, in turn, may cause changes (either structural or electrical) in the material that adversely affect its dielectric properties. Consequently, the present invention is suited for use in any semiconductor device which has a dielectric material that is susceptible to damage when penetrated by holes. In the prior art, the MOSFET gate dielectric layer is formed on a silicon substrate and the electrode formed on top of the gate dielectric layer is typically polycrystalline silicon. The device of the present invention creates an environment that reduces the number of holes that will penetrate the gate dielectric layer by placing a layer of high bandgap material adjacent to the dielectric layer.
In a MOSFET device, the gate dielectric layer is formed on a semiconductor substrate with source and drain regions formed therein. In the MOSFET embodiment of the present invention, the high bandgap material is formed over the gate dielectric layer (i.e. the dielectric layer is between the high bandgap layer and the substrate). The high bandgap material is electrically conductive. The remaining portion of the gate is then formed over the layer of high bandgap material.